The present invention is directed to a self-curing composition based on polymethyl methacrylate (PMMA) which can be used advantageously in bone surgery, as well as to a process for its production.
Self-curing synthetic resins have been known for a long time and have also been used for a long time in operative orthopedy. A material provided for implantation has very special demands placed on it, for example chemical and mechanical stability even under the long time effect of body fluids, biological compatibility, producibility in any desired form, good sterilizability, etc. At the same consideration should be given to the economy and cost worthiness in using such materials. Therefore it is not surprising that the selection of suitable products previously has been limited to only a few types of synthetic resins, of which polymethyl methacrylate has been chiefly used in orthopedic surgery.
Polymethyl methacrylate (PMMA) resins are hard, flexible, colorless, thermoplastic synthetic resins which also withstand severe mechanical loads and which because of their biological compatibility and good sterilizability are suitable for the production of elastic protheses. Thus, e.g. under the trademarks Paladon, Propolat and Palacos PMMA products have long been known as dental protheses synthetic resins or as bone cements for orthopedic surgery. In principle it is a matter of polymer powder which is stirred to a paste with the corresponding monomeric liquid (methyl methacrylate) which, according to the type of further additions, particularly polymerization catalysts hardens to a very solid, homogeneous composition at room temperature or at elevated temperature in a short time.
Although those PMMA products have been employed for a long time in bone surgery, their use is not completely without problems. While a skilled operator in the mixing in and employing of the PMMA bone cement cannot avoid, as the development of heat during the polymerization or the effect of the added MMA monomers on the circulation and the heart function of the patients, as a rule though suitable conditions it can be reduced to an acceptable composition for the patients, it particularly eliminates the procedures which accomplish the steering control of the operator in the bone cement and at the boundary of bone cement and protheses, particularly, however at the bone cement-bone boundary after the implantation. Thus a definite shrinkage of the known, previously employed PMMA polymers in the complete curing cannot be avoided, wherein the shrinkage of the monomer alone during the polymerization alone amounts to about 23% according to Loskaek and Fox, J. Amer. Chem. Soc. Vol. 75, page 3544 (1953). This shrinkage can permanently negatively influence the secure fastening of the protheses in the bone cement as well as the ratio at the bone cement-bone boundary and be important as the cause of numerous failures which occur with increasing frequency with increased interval from the time of implanting the protheses.
The present invention therefore is based on the problem of providing a self-curing synthetic composition which does not have the above mentioned disadvantages and therefor is better suited than previously known synthetic resins as bone cement in the orthopedic surgery.